Conventionally, a Rankine cycle has been used as one method for collecting and using exhaust heat energy dissipated into the atmosphere through exhaust gas or cooling water of an internal combustion engine. This Rankine cycle is a type of irreversible heat cycle as well as a theoretical cycle with a steam turbine, and includes a pump that pressure-feeds a working medium, a boiler (evaporator) that vaporizes the working medium with exhaust heat from an engine, a turbine (expander) that is driven with the working medium flowing out from the boiler to output power, and a condenser that liquefies the working medium flowing out from the turbine.
There is proposed a technique in which this Rankine cycle is provided to an automobile; water, ethyl alcohol with a low boiling point, a mixture of water and ethyl alcohol, or a chlorofluorocarbon substitute gas (for example, HFC134a, HFC245fa, or HFO1234yf) is used as the working medium; exhaust heat energy from an engine (internal combustion engine) is recovered; changes in volume at the time of vaporization are converted into power; and this power is used as assistance with an output of shaft from the engine or for generating electric power.
There is proposed a device having a Rankine cycle provided to an automobile, which includes: an evaporator that exchanges heat between an exhaust gas recirculation (EGR) gas and a working medium; a superheater that exchanges heat between the working medium flowing out from the evaporator and exhaust flowing out from a turbo charger; and an expander that is driven with the working medium flowing out from the superheater, and whereby recovers exhaust heat from the internal combustion engine (see, for example, Patent Literature 1). With this configuration, in the internal combustion engine including the turbo charger and an EGR system, exhaust heat loss is efficiently recovered, and is regenerated as power, thereby further improving thermal efficiency.
However, the output of power from the Rankine cycle uses exhaust heat from the engine, and depends on a state of operation of the engine. Thus, it is not possible to stably output the power in a state where exhaust heat energy is low, for example, immediately after the engine has started, immediately before the engine stops, or during a low-load operation, and an effect thereof on a practical fuel efficiency is merely several percents.
Meanwhile, there is a device having a steam accumulator as well as the Rankine cycle (see, for example, Patent Literature 2 and Patent Literature 3).
A device described in Patent Literature 2 accumulates steam in a steam accumulator during normal operation, stops a main engine during idling operation, and discharges steam from the steam accumulator to drive the Rankine engine, thereby resuming the main engine with the Rankine engine. With this configuration, fuel efficiency is improved in driving where the engine frequently performs the idling operation.
Furthermore, a device described in Patent Literature 3 performs regenerative power assist using regenerative power during fuel cut for an engine at the time of speed reduction, thereby suppressing an increase in the vehicle deceleration caused by auxiliary machine regeneration. With this configuration, the amount of regeneration with the auxiliary machine is increased, whereby fuel efficiency is improved. Furthermore, the steam accumulator is used to delay a timing of generating the regenerative power, whereby the regenerative power assist starts at a time when the vehicle deceleration reaches an allowable deceleration. Thus, it is possible to prevent excessive deceleration while achieving an appropriate sense of speed reduction at the beginning of speed reduction, which makes it possible for a driver to feel a natural speed reduction.
These devices store, in the steam accumulator, the exhaust heat energy from the engine, and control timings of outputting power so as to output the power when needed, thereby achieving an improvement in fuel efficiency. However, with these devices, the amount of exhaust heat energy that can be used is limited to the amount of the exhaust heat energy that the steam accumulator can store, and hence, the exhaust heat energy from the engine cannot be efficiently used. In addition, if the capacity of the steam accumulator is increased, mountability to a vehicle deteriorates accordingly.